This invention relates to novel modified starches and particularly amino-multicarboxylate starch derivatives and reagents used in the preparation thereof.
The modification of starch by chemical derivatization and, in particular, the preparation of starch ethers and esters is welI known in the art. The addition of cationic or anionic substituent groups to form cationic or anionic starch derivatives is also well known. A discussion of the preparation of cationic or anionic starch derivatives may be found in "Starch: Chemistry and Technology", Second Edition, edited by R. L. Whistler, et al., 1984, pp. 332-364.
It is further known to add both cationic and anionic substituent groups to starch to form amphoteric starch ether derivatives. Typical prior art methods involve either an "in situ" formation of the amphoteric starch ether derivatives by simultaneous reaction of two different chemical modifying agents with the starch as described in U.S. Pat. Nos. 3,459,632 and 4,119,487 or a multi-step procedure where the chemical modifications of the starch are carried out in sequence as described in U.S. Pat. No. 3,751,422.
It is also known to add mono-cationic and mono-anionic substituent groups to the same reactive site in the starch molecule, thereby forming zwitterion starch ether derivatives. The preparation of a zwitterion starch ether derivative, wherein the zwitterion substituent groups are amino-monocarboxylic acid groups, is described in U.S. Pat. No. 4,017,460. In this patent the derivatives are prepared by reacting the starch base with a reagent obtained by reacting a secondary amine with a dihalopropionic acid or alkyl ester thereof. The resulting zwitterion group contains one anionic carboxyl group bound directly to a cationic amine group. Another zwitterion starch ether derivative is disclosed in U.S. Pat. No. 4,260,738 where the zwitterion substituent groups are aminophosphonic acids or their salts.
In contrast to the amino-monocarboxylic acid zwitterion starch ethers described in the aforementioned U.S. Pat. No. 4,017,460, the novel starch derivatives of this invention contain amino-multicarboxylate groups as the amphoteric or zwitterion substituent group. These amino-multicarboxylate starch derivatives possess different charge ratios and exhibit properties making them especially useful in papermaking as retention and strength aids and as metal chelating agents.